As densities and complexities of integrated circuits (ICs) continue to increase, inspecting photomask patterns become progressively more challenging. Every new generation of ICs has denser and more complex patterns that currently reach and exceed optical limitations of lithographic systems. To overcome these limitations, various Resolution Enhancement Techniques (RET), such as Optical Proximity Correction (OPC), have been introduced. For example, OPC helps to overcome some diffraction limitations by modifying photomask patterns such that the resulting printed patterns correspond to the original desired patterns. Such modifications can include perturbations to sizes and edges of main IC features, i.e., printable features. Other modifications involve additions of serifs to pattern corners and/or providing nearby sub-resolution assist features (SRAFs), which are not expected to result in printed features and, therefore, are referred to as non-printable features. These non-printable features are expected to cancel pattern perturbations that would otherwise have occurred during the printing process.
Adding non-printable features to a mask having a dense population of printable features further complicates photomask inspection. Furthermore, non-printable and printable features have different impacts on the printed pattern. As such, defects corresponding to these features will have different lithographical significance, and some of the defects can be ignored during inspection.